1. Technical Field
The present invention relates to wireless communication, and more particularly, to a method for performing cell search in a wireless communication system.
2. Related Art
Wide code division multiple access (WCDMA) systems of the 3rd generation partnership project (3GPP) use a total of 512 long pseudo-noise (PN) scrambling codes in order to identify base stations (BSs). As a scrambling code of a downlink channel, each BS uses a different long PN scrambling code.
When power is supplied to a user equipment (UE), the UE performs downlink synchronization of a cell and acquires a long PN scrambling code identifier (ID) of the cell. Such a process is generally referred to as a cell search. The cell search is the procedure by which a user equipment acquires time and frequency synchronization with a cell and detects the cell identity of the cell. The initial cell is determined according to a location of the UE at a time when the power is supplied. In general, the initial cell indicates a cell of a BS corresponding to the greatest one of signal components of all BSs, which are included in a downlink reception signal of the UE.
To facilitate the cell search, a WCDMA system divides 512 long PN scrambling codes into 64 code groups, and uses a downlink channel including a primary synchronization channel (P-SCH) and a secondary synchronization channel (S-SCH). The P-SCH is used to allow the UE to acquire slot synchronization. The S-SCH is used to allow the UE to acquire frame synchronization and a scrambling code group.
In general, cell search is classified into initial cell search, which is initially performed when a UE is powered on, and non-initial search which performs handover or neighbor cell measurement.
In the WCDMA system, the cell search is accomplished in three steps. In the first step, a UE acquires slot synchronization by using a P-SCH including a primary synchronization code (PSC). A frame includes 15 slots, and each BS transmits the frame by including a PSC. Herein, the same PSC is used for the 15 slots, and all BSs use the same PSC. The UE acquires the slot synchronization by using a matched filter suitable for the PSC. In the second step, a long PN scrambling code group and frame synchronization are acquired by using the slot synchronization and also by using a S-SCH including a secondary synchronization code (SSC). In the third step, by using a common pilot channel code correlator on the basis of the frame synchronization and the long PN scrambling code group, the UE detects a long PN scrambling code ID corresponding to a long PN scrambling code used by the initial cell. That is, since 8 long PN scrambling codes are mapped to one long PN scrambling code group, the UE computes correlation values of all of the 8 long PN scrambling codes belonging to a code group of the UE. On the basis of the computation result, the UE detects the long PN scrambling code ID of the initial cell.
Since the WCDMA system is an asynchronous system, only one PSC is used in the P-SCH. However, considering that a next generation wireless communication system has to support both synchronous and asynchronous modes, there is a need for using a plurality of PSCs.
If errors occur while detecting the S-SCH, delay occurs when a UE performs cell search. Therefore, there is a need to improve channel detection performance in the cell search procedure.